1. Field of the Invention
This invention is related to electron-beam lithography. In particular, it is related to proximity-effect correction for raster-scan or vector scan electron-beam lithographic systems.
2. Prior Art
Electron-beam technology can be used for producing patterns in large scale integrated (LSI) circuit manufacturing, or for producing masks to be used in optical or X-ray lithography. A layer of photoresist material, deposited on a substrate or over a previously formed structure, is exposed by an electron beam to form the desired pattern by subsequent developing, etching and depositing or removing further material to obtain a structure of fine metal, semiconductor or insulator patterns having specific properties.
The electron beam can expose the photoresist either through a mask, or it can be guided over the photoresist in a raster-scan or vector-scan method to generate the desired pattern. Such electron beam lithography systems are described, e.g. in U.S. Pat. Nos. 3,644,700, 3,866,013, 3,875,415, 3,876,883, and 4,056,730.
An undesirable effect in electron-beam lithography is exposure by backscattered electrons. This results in nonuniform developing of exposed resist areas, depending on the shape and interrelation of the generated patterns. Fine lines, for example, tend to be underdeveloped and gaps between exposed areas tend to become smaller than desired. Conversely, in corners or in the center of wide lines, some overdeveloping occurs, which is also designated as blooming. The undesired exposure by backscattered electrons and the resulting non-uniform developing are generally called "proximity effects" because they occur in the proximity of the areas exposed by the electron beam (which causes the backscattered electrons). The amount or intensity of the backscattering depends on the thickness and composition of the photoresist layer and on the substrate material, and its distribution is dependent on shapes or topological detail. Proximity effects may occur intrashape, for example, within a single line if it is wide enough or makes turns, or they may occur intershape, that is, electron beam exposure for one line may have an influence on the exposure extent of a line located in close proximity but covered by the electron beam earlier or later (in another scan pass).
Methods have been suggested and are used by which proximity effects in electron beam lithography can be reduced or eliminated. Such methods were disclosed in following publications:
M. Parikh: "Proximity effects in Electron Lithography: Magnitude and Correction Techniques", IBM J. of Res. and Dev., Vol. 24, No. 4, July 1980, pp. 438-451.
W. D. Grobman et al: "Proximity Correction Enhancements for 1-.mu.m Dense Circuits", IBM J. of Res. and Dev., Vol. 24, No. 5, September 1980, pp. 537-544.
T. P. Chang et al: "Partitioning E-Beam Data for Proximity Corrections", IBM TDB, Vol. 20, No. 9, February 1978, pp. 3809-3812.
M. Parikh: "Self-Consistent Correction of Proximity Effects in Electron-Beam Lithography", IBM TDB, Vol. 22, No. 9, February 1980, pp. 4327-4328.
E. Bretscher: "Proximity Correction in Electron Beam Lithography", IBM TDB, Vol. 23, No. 6, November 1980, p. 2541.
These methods essentially involve the following considerations. The shapes to be generated are analyzed and subdivided to obtain areas of equal backscatter electron distribution. An algorithm is used to determine, by a computer program in advance of the lithographic procedure, how an even dose distribution of electrons for the given shape can be achieved despite proximity effects. The electron beam is then controlled by the program output data in a different way than it would be controlled without correction. This may involve reduction of shapes to reduce excessive exposure by backscatter electrons, and expansion of shapes which otherwise would not be developed sufficiently in relation to other portions of a pattern. Alternatively, it may involve subdividing the shape into different areas for which different electron beam dwell times or intensities are used.
It is a disadvantage of these known methods that they need preprocessing. That is, an analysis must be made in advance of the electron beam exposure process. Furthermore, these methods are based on approximative models and rely on presumed dimension parameters which actually may be slightly different.